1. Field
The described embodiments and claimed invention relate to a tool for sequentially engaging and releasing a restrictor element, also referred to as plug, onto and from its corresponding valve seat, as well as systems and methods incorporating such a tool for producing hydrocarbons from multiple stages in a hydrocarbon production well.
2. Background of the Art
In hydrocarbon wells, tools incorporating valve assemblies having a restrictor element, such as a ball or dart, and a seat element, such as a ball seat or dart seat, have been used for a number of different operations. Such valve assemblies prevent the flow of fluid past the assembly and, with the application of a desired pressure, can actuate one or more tools associated with the assembly.
One use for such remotely operated valve assemblies is in fracturing (or “fracing”), a technique used by well operators to create and/or extend one or more cracks, called “fractures” from the wellbore deeper into the surrounding formation in order to improve the flow of formation fluids into the wellbore. Fracing is typically accomplished by injecting fluids from the surface, through the wellbore, and into the formation at high pressure to create the fractures and to force them to both open wider and to extend further. In many case, the injected fluids contain a granular material, such as sand, which functions to hold the fracture open after the fluid pressure is reduced.
Fracing multiple-stage production wells requires selective actuation of valve assemblies, such as fracing sleeves, to control fluid flow from the tubing string to the formation. For example, U.S. Published Application No. 2008/0302538, entitled Cemented Open Hole Selective Fracing System and which is incorporated by reference herein, describes one system for selectively actuating a fracing sleeve that incorporates a shifting tool. The tool is run into the tubing string and engages with a profile within the interior of the valve. An inner sleeve may then be moved to an open position to allow fracing or to a closed position to prevent fluid flow to or from the formation.
That same application describes a system using multiple valve assemblies which incorporate ball-and-seat seals, each having a differently-sized ball seat and corresponding ball. Frac valves connected to ball and seat seals do not require the running of a shifting tool thousands of feet into the tubing string and are simpler to actuate than frac valves requiring such shifting tools. Such ball and seat seals are operated by placing an appropriately sized ball into the well bore and bringing the ball into contact with a corresponding ball seat. The ball engages on a sealing section of the ball seat to block the flow of fluids past the valve assembly. Application of pressure to the valve assembly causes the valve assembly to “shift”, opening the frac sleeve.
Some valve assemblies are selected for tool actuation by the size of ball or other restrictor element introduced into the well. If the well or tubing string contains multiple ball seats, the ball must be small enough that it will not seal against any of the ball seats it encounters prior to reaching the desired ball seat. For this reason, the smallest ball to be used for the planned operation is the first ball placed into the well or tubing and the smallest ball seat is positioned in the well or tubing the furthest from the wellhead. Thus, these traditional valve assemblies limit the number of valves that can be used in a given tubing string because each ball size is only able to actuate a single valve. Further, systems using these valve assemblies typically require each ball to be at least 0.125 inches larger than the immediately preceding ball. Therefore, the size of the liner restricts the number of valve assemblies with differently-sized ball seats. Certain seat assemblies may allow plug increments of 0.0625 inches, which provides more available seats, but still creates an upper limit on the total available plug sizes. In other words, because a plug must be larger than its corresponding plug seat and smaller than the plug seats of all upwell valves, each plug can only seal against a single plug seat and, if desired, actuate one tool.
The valve assembly provides a method for sequentially sealing multiple valve seats with a single restrictor element and, where desired, actuating tools associated with the valve assembly. One embodiment allows multiple balls, plugs or other restrictor elements of the same size to actuate tools in sequential stages.